Phase relationships of S-type granite with H2_O to 35 kbar: Muscovite granite from Harney Peak, South Dakota

Abstract

Muscovite granite (13.8% muscovite, 4.8% normative corundum) was reacted, with varying percentages of H_2O, in cold-seal vessels at 2 kbar and in piston-cylinder apparatus between 10 and 35 kbar. The diagrams illustrating melting/crystallization relationships are: P-T sections with both excess H_2O and with no H_2O added (0.66% H_2O in rock); T-X_(H_2O) sections at 15 kbar and 25 kbar showing H_2O-undersaturated conditions; the H_2O-undersaturated surface for the crystallization of quartz/coesite (small amounts of aluminosilicate minerals persist to higher temperatures). Glass compositions measured by electron microprobe from samples with 5% H_2O at 15 kbar confirm that liquids are syenitic through at least 100°C above the solidus, as predicted from the effect of pressure on the Residua System. Results are explained successfully by phase relationships involving muscovite, quartz, and orthoclase in K_2O-Al_2O_3-SiO_2-H_2O, with reactions depicted in a P_(total)-P_(e_(H_2O))-T model, with special reference to the divariant surfaces in the region P_(e_(H_2O)) < P_(total). With reduced P_(e_(H_2O)) (or a_(H_2O)), produced either by small amounts of H_2O (and H_2O-undersaturation) or by CO_2 + H_2O mixtures, subsolidus dehydration reaction temperatures decrease, vapor-present solidus temperatures increase, and muscovite stability in presence of liquid increases. In general, muscovite, biotite, and amphibole can be precipitated from magmas containing only a few tenths per cent H_2O (although the H_2O-undersaturated liquids coexisting with crystals may contain 3% or more dissolved H_2O). This particular granite cannot be a primary magma from mantle or subducted oceanic crust. It is a possible product of partial fusion of pelitic rocks between about 20 km and 40 km depth given sufficient H2O, and xenocrystal muscovite or sillimanite from the source rocks. The phase relationships are consistent with the idea of S-type granites, but not sufficient to prove the origin of this rock. Additional tests require phase relationships of other associated granitic rocks, and details of geochemistry, geophysics, and field relationships.

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